The present invention relates generally to optical low-pass filters, and more particularly to an optical low-pass filter used in a color separating optical system of an image pickup tube in a television camera, which is of a simple construction and does not require positional adjustment.
In a television camera which uses a single image pick-up tube, a color separating optical system including a color stripe filter is used. However, in this optical system, beat is introduced between the high-frequency component of the luminance signal and the chrominance signal. Accordingly, in order to reduce the high-frequency component of the luminance signal so as to reduce the above beat, and optical low-pass filter (generally a filter having an attenuation pole) is generally employed.
A conventional optical filter of the above type used a polyhedron prism, for example. This conventional low-pass filter is unitarily formed from a polyhedron prism in which a plurality of surfaces are arranged radially from a vertex of the polyhedron in an oblique manner.
However, the shape of the above conventional low-pass filter is complex, and is disadvantageous in that it is difficult to manufacture the optical low-pass filter, and the cost of the filter is high. Furthermore, the optical low-pass effect decreases when the above optical low-pass filter is not mounted at the correct position with respect to an iris within the optical system. Accordingly, the vertex of the optical low-pass filter must be positioned and mounted so that the vertex accurately coincides with the center position of the iris within the optical system, and was disadvantageous in that the assembling and adjusting operation are troublesome.
Moreover, in an another example of a conventional optical low-pass filter, a quartz crystal plate was used. However, in this type of a conventional optical low-pass filter, the cost of the filter became high because the quartz crystal used is expensive. In addition, in such cases where the incoming beam is linearly polarized or when a plurality of quartz crystals are used, phase plates for one-quarter wavelength (.lambda./4 plate) and the like are required to obtain circular polarization. Therefore, the optical system became complex, and the optical low-pass filter could not be manufactured at low cost.